Three longitudinal studies are proposed that build on previous research conducted through CPEA funding. The first two longitudinal studies address issues in the frequency and associated features of behavior problems and adolescent onset of seizures in 257 11-18 year-old children and adolescents, either referred for possible autism during preschool years or identified with developmental disabilities in a matched control group for a subset of the children with possible autism. These studies will be conducted at the University of Michigan, using samples from previous research carried out by Drs. Lord and Cook in Chicago and North Carolina, and a sample from Yale. Major hypotheses are that the predominant trajectory for behavior problems will be a decrease in irritability and hyperactivity in the year following onset of puberty, except in cases of onset of seizures and in a small minority of children who already had significant behavioral difficulties. In addition, a model of the relationship between child factors, including history and current behavior difficulties, severity of symptoms of autism and cognitive and language delay, parental perceptions of the burden of having a child with autism, and parental feelings of well-being is proposed, with intervening variables of parental accommodations to difficulties and amount of treatment. The third study involves comparison of follow-up data for five year-olds seen at the Yale Developmental Disabilities Clinic because of possible autism to data from two cohorts followed from age two in North Carolina and Chicago. Trajectories of language development and increases in adaptive behavior will be compared across cohorts, as will the stability of early diagnosis, using both standardized (ADI-R, ADOS) and clinical best estimate procedures.